1. Technical Field
This invention relates to a voltage controlled oscillator (VCO) circuit, and particularly to an integrated circuit (IC) implemented VCO wherein ratios of IC component values are employed to produce an output signal having a frequency proportional to the voltage of a control signal.
2. Background Art
Circuit components on an IC have very poor actual tolerances from one IC to the next. Specifically, the capacitance of a capacitor, or the resistance of a resistor could vary as much as .+-.20%. For instance, suppose it was desired that an IC have a 10 ohm resistor as part of the circuit implemented thereon. After fabrication, this resistor on one IC chip in a lot may have the desired 10 ohm resistance, while the same resistor on other like chips could exhibit resistance anywhere from 8 ohms to 12 ohms (i.e. .+-.20% variance). Consequently, the circuit from one IC chip to the next could exhibit significantly different characteristics. In the case of a VCO, this above-described variance is particularly problematic. Many well known VCO circuits rely on a resistor and capacitor to set the frequency of the output signal to a specific level based on the voltage of a control signal. If the resistance and capacitance of these respective components were allowed to vary up to .+-.20% between IC chips, then worst case, the frequency of the output signal at a given control signal voltage could vary up to .+-.40%. This level of variance is clearly unacceptable for mass produced electronic devices employing a VCO circuit. As a result, a variety of external circuitry has been required in the past to ensure one IC chip exhibited the same frequency output characteristics as another.
It is, therefore, an object of the present invention to provide the architecture for an integrated VCO circuit whose output frequency does not vary significantly from one IC chip to the next, for a given control signal voltage input.
It is further object of the present invention to provide the architecture for an integrated VCO circuit which does not require external circuitry for consistency between IC chips.
And, it is still another object of the present invention to provide the architecture for an integrated VCO circuit which accomplishes a close matching of characteristics between IC chips by employing the phenomenon whereby IC component resistance and capacitance value ratios do not vary significantly from one IC chip to another.
Other objects and benefits of the invention will become apparent from the detailed description which follows hereinafter when taken in conjunction with the drawing figures which accompany it.